1. Field of the Invention
The present invention provides a method for detecting a tone signal corresponding to a tone frequency, and more specifically, to a method applying digital signal processing and a zero-crossing rate operation for detecting a tone signal.
2. Description of the Prior Art
A continuous tone-coded squelch system (CTCSS) has been broadly applied in the radio communication industry for communication between users through the same channel in a communication area. A CTCSS applies a low frequency tone signal (CTCSS tone) to distinguish a signal transmitted on a channel. A prior art walkie-talkie applies a CTCSS for group communication.
Please refer to FIG. 1. Illustrated in FIG. 1 is a graph of frequency range of a prior art CTCSS. A frequency range between 62.5 Hz and 250 Hz is applied for transmitting a low frequency tone signal, and a frequency range between 300 Hz and 3.4 KHz is applied for transmitting a speech signal generated by a user. In a prior art walkie-talkie, 14 physical channels, labeled from P1 to P14, are defined for transmitting and receiving signals. Each physical channel can be divided into 38 logical channels, labeled from T1 to T38, according to 38 different frequency tone signals so that 532 (14*38) logical channels are available. In a practical operation, a person wishing to communicate sets up a logical channel of a walkie-talkie, and pushes a push-to-talk (PTT) button for outputting his speech signal to a predetermined communication area. A walkie-talkie of a receiver in the predetermined communication area first determines whether or not the transmitted signal is on the same physical channel. If the physical channel is the same, the receiver's walkie-talkie starts to receive the speech signal and determines whether or not the transmitted speech signal has the same tone signal. If the physical channel is not the same, the walkie-talkie does not receive the speech signal. If the tone signal is the same, meaning that the logical channels of the two walkie-talkies are the same, the received speech signal is output through a speaker; if not, the walkie-talkie performs a squelch function and does not output the received speech signal. Only people in the predetermined communication area having walkie-talkies on the same logical channel can communicate as a group.
How to detect a logical channel is an important issue in a prior art walkie-talkie. Please refer to FIGS. 1 and 2. Illustrated in FIG. 2 is a functional block diagram of a prior art walkie-talkie. A portable walkie-talkie 10 comprises an antenna 11, a transceiver 12, a selector 14, a CTCSS decoder 16, a CTCSS encoder 18, a speech signal processing unit 20, a speaker 22, a microphone 24, and a control unit 26. A high frequency RF signal is received and transmitted through the antenna 11 of the walkie-talkie 10, converted to a low frequency Rx signal by the transceiver 12, transmitted to the selector 14, and output from point A of the selector 14 to the CTCSS decoder 16. The CTCSS decoder 16 comprises an analog signal frequency filtering circuit having an operational frequency range between 62.5 Hz and 250 Hz. The CTCSS decoder 16 identifies the frequency of a tone signal of the Rx signal, determines whether or not the tone signal of the Rx signal belongs to the same logical channel, and then outputs an analytical result to the control unit 26. If the corresponding logical channel of the Rx signal is the same as that of the walkie-talkie 10, the control unit 26 turns on the speech signal processing unit 20 causing the speaker 22 to output a speech signal. The speech signal processing unit 20 comprises two analog signal frequency filtering circuits having an operational frequency range from 300 Hz to 3.4 KHz. In contrast, if the corresponding logical channel of the Rx is different than that of the portable walkie-talkie, the control unit 26 does not turn on the speech signal processing unit 20 and the speaker 22 so that no speech signals are output. When a user pushes a PTT button, the selector 14 selects the point B, and the control unit 26 turns on the microphone 24 so that a speech signal of the user is transmitted through the speech signal processing unit 20 to the encoder 18. The encoder 18 then adds a corresponding tone signal to the output signal from the speech signal processing unit 20 according to the logical channel of the portable walkie-talkie 10, and generates a Tx signal that is converted to a high frequency RF signal and output through the antenna 11.
The prior art walkie-talkie 10 applies analog signal processing to tone signals for processes ranging from receiving an analog RF signal to outputting an analog Rx signal. A CTCSS decoder 16 must use a prior art frequency filtering circuit for collecting signals from the 62.5 Hz to 250 Hz frequency range. However, the prior art frequency filtering circuit cannot concisely collect signals from such a frequency range due to inherent limitations. As 38 tone signals are positioned from 62.5 Hz to 250 Hz, a frequency difference between neighboring tone signals is only about 3 Hz to 5 Hz. Thus a CTCSS decoder 16 may fail to properly distinguish a tone signal.